Backlash may potentially occur in a vehicle driveline when the torque transmitted through a mechanical contact (such as a set of gears) changes from positive to negative, or vice versa (ignoring acceleration effects). For example, when a vehicle is decelerating, the compression braking effect of the engine provides negative torque to the transmission which is then passed through the differential and then to the wheels. At this point, the driveline is wrapped in the negative direction. If the vehicle operator steps into the accelerator pedal, the transmitted engine torque switches from negative to positive as it begins to supply torque to propel the vehicle forward. The driveline unwraps as each driveline component changes from transmitting negative torque to transmitting positive torque. At some point during the transition, the entire driveline may be considered to be in a relaxed state with zero torque applied to the wheels. During this zero torque region, gear teeth in the transmission and other driveline elements may not be tightly coupled to their mating gears. As the engine continues to provide positive torque, the driveline will wrap in the positive direction. The gears are then quickly coupled to the corresponding mating gears which produces a “clunk”. The driveline clunk may be sensed by the vehicle operator and the vehicle operator may perceive the clunk to objectionable
In one approach, driveline backlash and clunk may be reduced by adjusting engine torque during a transition in torque transmission direction. In particular, the engine torque (which can be considered on the upstream side of the lash region) may be adjusted substantially slowly through the zero torque region of the transition in order to reduce wrapping of the driveline. Further, in a hybrid vehicle system, such as described in U.S. Pat. No. 6,574,535 for example, motor torque may be adjusted on the upstream side of the lash region to provide accurate control of the slower torque transition near the lash region to further reduce clunk.
However, the inventors herein have recognized some issues relating to the above approach. Specifically, in some conditions, since the propulsion source(s) are located upstream of the transmission and the gear elements where clunk may occur, the propulsion sources are slowly adjusted, especially through the zero torque region, in order to inhibit driveline backlash and clunk. By slowing the transition of torque transmission direction of the propulsion source, torque response to the wheels is also delayed. While various approaches can be used to minimize this delay, the delay is the fundamental features used to reduce clunk. As such, vehicle response may, in some cases, be perceived as sluggish by the vehicle operator.
In at least one approach described herein, at least some of the above issues may be addressed by a hybrid electric vehicle comprising: an internal combustion engine; a mechanical torque transmission device for transmitting engine torque to at least one wheel, the mechanical torque transmission device having a lash region; an electric energy conversion device connected downstream of the mechanical torque transmission device; and a control system, the control system, adjusting the electric energy conversion device to meet a desired vehicle response, adjusting the internal combustion engine torque to transition through the lash region, and then adjusting the electric energy conversion device torque and the internal combustion engine torque to meet the desired vehicle response. In this way, the hybrid vehicle may be controlled to provide a quick response to driver demand while reducing the NVH effects of driveline backlash. In other words, the smooth transition through the lash region can be provided without delaying the vehicle response since the motor is downstream of the lash region, and therefore can provide a rapid vehicle response without requiring a transition through the lash region. Rather, the transition may occur on a slower scale since it may be decoupled from the vehicle response.
Furthermore, the hybrid vehicle operating efficiency may be improved by performing engine and electric energy conversion device control according to the state of charge of the battery.
As another approach described herein, at least some of the above issues may be addressed by a method for inhibiting driveline backlash in a hybrid electric vehicle including an internal combustion engine providing torque along a driveline via a transmission, and an electric energy conversion device located downstream of the transmission configured to provide torque to the driveline is provided, the method comprising: adjusting the downstream electric energy conversion device to provide zero torque output during a zero torque region of a transition in engine torque transmission direction; and then adjusting the downstream electric energy conversion device to provide torque output corresponding to internal combustion engine torque output.
Since the electric energy conversion device is located downstream of the engine and transmission near the wheels, the electric energy conversion device may be used to buffer and control any sudden torque change between the engine and the wheels. Further, because the electric energy conversion device has very fast transient response, it can be used to prevent the driveline from storing energy and causing a spring effect. In this way, sudden changes between engine torque and wheel torque may be buffered, in turn, diminishing energy stored in the driveline that facilitates a reduction in driveline NVH and clunk.